I just wish there was a PS5 equivalent for launch probably like Series X will have but i'm also more than happy it's not proprietary which means there's more options out there down the line and perhaps sizes way bigger than keeping it proprietary.

Well, "not proprietary" doesn't necessarily equal affordable, have you seen Samsung SSD prices? Also, since the PS5 SSDs have 6 priority lanes vs NVMe 2 priority lanes so it kind of is proprietary, I wouldn't expect it to be more affordable than the customized XSX SSDs.

I wonder if the approved PS5 SSDs will all be multiples of 825GB, and I also wonder how that was the "natural" size for storage? Maybe that was answered in the other forum.

PS5 uses PCIe 4.0 so it's faster than the PCIe 3.0 that Xbox uses which is 10 years old already, hence they will need SSDs that support that speed on PS5 that's why they are telling people to only buy certified SSDs that are capable of hitting the transfer rates that are needed.

It's a brand new interface so there's pretty much nothing on the market right now. There may be like 4 or 5 raging from 500gb to 2tb £120 for 500gb - £445 for 2tb.

By the time PS5 launches there should be a lot more available as well as certified versions guaranteed to work inside the PS5.

The SSDs Xbox is using are based on the old standard that is already available. So I would hope that it would be no more than £100 for a 1TB expansion.

It's not so much the PCIe 4.0, it's that Sony is using PCIe 4.0 compatible SSDs w/ 6 priority lanes, above the minimum spec of 2 priority lanes. It is still 4 PCIe 4.0 lanes though.
Comparing various versions of a 1TB PCIe 3.0 M.2 NVMe SSD from Samsung:

970 EVO = $180
970 EVO Plus = $230
970 EVO Pro = $320

Of course none of this tells us how much MS will charge for their SSD, or how much Samsung will charge for their 4.0 versions, or which tier will be fully compatible w/ PS5, it is only an example.

Also, this article says the XSX uses a "Custom NVMe PCI-e 4.0" but the specs match 3.0 and all other articles (and MS' official sites) just say "custom NVMe," so Windows Central is probably wrong...

It would be a nice surprise if it really is 4.0. Obviously AMD supports it since they worked w/ Sony on their custom I/O, so perhaps MS' Velocity Architecture really is PCIe 4.0